The Assignee of the present invention manufactures CSDs and IDGs for use in generating 400 Hz. three-phase alternating current in airframes. FIGS. 1 and 2 illustrate conceptually a block diagram of a CSD or IDG of the type manufactured by the Assignee of the present invention. With reference to FIGS. 1 and 2 in which like parts are identified by like reference numerals, a variable speed power take-off 10 is driven by an airframe propulsion engine which varies in speed in direct proportion to the speed of the propulsion engine. When the unit of FIGS. 1 and 2 is an IDG, the power takeoff 10 drives a hydromechanical transmission 18 comprised of a hydraulic pump and motor (not illustrated) contained within the case 12. The transmission adds or subtracts shaft RPM from the power take-off 10 to produce a constant velocity output on shaft 20 which drives a three-phase generator 22 at a constant velocity to produce 400 Hz. three-phase alternating current. When the unit of FIGS. 1 and 2 is a CSD, the input drives a hydromechanical transmission within the case 12 (not illustrated) comprised of a hydraulic pump and motor (not illustrated) to produce a constant velocity output on shaft 14 which drives a three phase generator 16 at a constant velocity to produce 400 Hz. three phase alternating current. An IDG differs from the CSD in that the constant speed drive transmission 18, constant speed shaft output 20 and three-phase generator 22 are each contained within the case 12 instead of the generator 16 of the CSD being external of the case 12 of the transmission. The overall function of a CSD and an IDG is to perform the identical function of generating three-phase 400 Hz. electrical power in an airframe.
New IDGs operate at elevated temperatures. Ester based synthetic oils are used in the hydraulic pump and motor of the constant speed drive transmission in both a CSD and an IDG. New airframes have IDGs which produce a high electrical power output requiring the IDG to dissipate heat as a consequence of losses in the constant speed drive transmission and the generator.
The operation with synthetic ester based oils causes the formation of organic acids. A chemical reaction occurs with organic acids within the case 12 of a IDG or CSD to form a metallo-organic soap and/or insoluble products which circulate within the oil contained with the case of the CSD or IDG.
The CSD or IDG contains an oil circuit 24 which contains a filter 26 external to the case as illustrated in FIG. 1 or internal to the case as illustrated in FIG. 2. The oil circuit 24 containing the filter 26 cools the oil within the CSD or IDG. The oil circuit 24 additionally includes an oil pump 28 which scavenges oil within the case 12 to pump the oil 30 to the filter 26 where the oil is filtered to remove entailed solids. The output of the filter 26 is applied to an oil cooler 28 which dissipates the heat picked up by the oil from operation of the hydraulic pump and motor contained within the transmission of the CSD and the hydraulic pump and motor plus electric generator of the IDG.
The reaction of ester based synthetic lubricants to form organic acids in the oil 30 contained within the case 12 causes the metallo-organic soaps and/or the insoluble products to plug the filter 26 in a relatively short time of operation of the transmission such as 750 hours. Some aircraft have a specification of a minimum time of 1,200 hours between change of the filter 26 and the oil 30 which is not met by use of synthetic based oils which are approved generally for airframe applications such as MIL-L23699 and MIL-L7808.
The aforementioned synthetic oils, which are typically used in a CSD or IDG, do not contain adequate additives to prevent the formation of metallo-organic soaps and/or solids during the operational temperatures encountered by newer IDGs which cause the filter to become plugged or otherwise not fully operational requiring replacement sooner than its specified service life. The specifications of the aforementioned oils do not require sale with additives especially suited for preventing the formation of metallo-organic soaps and/or solids within a CSD or IDG. As a consequence of the overall relatively small quantity of oil which is sold for use in the CSD or IDG transmissions, no manufacturer of oil has been willing to conduct the necessary tests to obtain approval of a synthetic based oil containing adequate additives for preventing the formation of the aforementioned metallo-organic soaps and solids to permit the filter and oil to be used for their specified service life. The currently available synthetic ester based used oils break down into organic acids that chemically attack the metal casing 12 of the CSD or IDG. Formulating new or revised oils that will not form organic acids and/or attack the CSD or IDG metal casing would require extensive field/flight evaluations with approval taking from two to eight years with the norm being around six years. The Assignee has requested oil companies to develop a specific IDG/CSD oil for nine years without obtaining any interest on the part of oil companies to do so.
Liquid oil additives are known which may be mixed with synthetic oils to neutralize acids. For example, see U.S. Pat. Nos. 2,889,338, 3,941,709, 3,969,254, 3,976,585, 4,189,388, 4,226,732, 4,461,713, 4,568,474 and 4,943,383.
Servicing of a CSD and IDG includes regular oil and filter changes. Those regular oil and filter changes include changing the filter 26 and draining the oil 30 from the case 12. An airplane mechanic places a new filter in the CSD or IDG and fills the case with new oil 30. As a consequence of the problem involving formation of metallo-organic soaps and/or insoluble solids especially in newer IDGs, the recommended service intervals are so short as to cause airlines to want to have longer service intervals between routine oil and filter changes to lessen the overall operational costs of CSDs or IDGs.
The airlines desire an inexpensive oil filter which provides longer service intervals such as up to 3,0000 hours in order to lessen the operational cost of the CSD or IDG. A filter having a service life of 3,000 hours is currently not available as a result of the problems of the prior art discussed above.
FIGS. 3 and 4 illustrate a prior art oil filter of the type illustrated in FIGS. 1 and 2 which is used in a CSD or IDG. The oil filter 26 has an end cap 40 which has an outlet for discharging oil pumped under pressure by the oil pump 28. An O-ring seal 42 provides suitable sealing between a fitting mating with the end cap 40. Oil flows into the cylindrical chamber 44 within the filter 26 by flow radially inward through a pleated filtering media 46 as described below, an inner perforated cylindrical support tube 50 having apertures 52. The pleated filtering media 46 surrounds the inner perforated support tube 50 and prevents radially inward deflection of the filtering media 46 caused by radial inward flow of pressurized oil. The pleated filtering media 46 contains interstices for trapping solid particles flowing within the oil. Oil flows inward through the filtering media 46, inner perforated support tube 50 and from the outlet at the end cap 40 to the oil circuit 24. The filtering media 46 and inner support tube 50 are glued to end cap 54 by a suitable adhesive such as epoxy glue. Similarly, end cap 52 is attached to the filtering media 48 and inner support tube by epoxy glue.
The filtering media 46 is comprised of a sandwich of four pleated layers illustrated in FIG. 3 and in detail in FIG. 4. The media 46 is formed into a cylinder 56 with pleats 58 extending longitudinally along the length of the filter. The cylinder 56 completely encircles the cylindrical chamber 44. The inner layer 60 (with reference to the cylindrical chamber 44) is an aluminum, stainless, or other metal type screen. A first intermediate layer 62, which is a nylon or polyester scrim, contacts the inner layer 56. A second intermediate layer 64, which is a fiberglass filtering media, contacts the nylon or polyester scrim 62. The outer layer 66, which is an aluminum, stainless steel or other metal type screen, contacts the fiberglass filtering media 64. The thickness and surface area of the filtering media determine the filtration produced by the filter 26. The aforementioned soaps and/or solids occlude the interstices of the filter.
In the prior art, the filter 26 is packaged dry within a suitable package for preventing its exposure to dirt. The airplane mechanic removes the filter 26 from the package and places it within the oil filtration circuit 24 while the filter 26 is dry. No additives are contained within the filter in the prior art. Oil is added to the case 12 during changing of the oil or to top off the case between oil changes.
U.S. Pat. Nos. 2,392,901, 2,785,805, 3,224,592 and 4,886,599 each disclose filters which have been coated with solids to neutralize the formation of acids within the oil being filtered. None of the aforementioned patents suggest the usage of a liquid additive which wets the interstices of the filter prior to use with a quality of liquid oil additive that is washed from the interstices into solution with the oil during filtration by the filter to form a mixture of the oil and additive which reduces the formation of metallo-organic soaps and solids that are retained within the interstices of the filter.